Membrane structures of conventionally known hollow fiber membranes include (1) membranes which comprise an inside surface and an outside surface both made of a skin layer and an intermediate spongy or finger-like structure therebetween, as described in, for example, JP-A-56-105704 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-56-115602, JP-A-58-132111, JP-A-58-1156018, JP-A-60-246812 and JP-A-61-164602 and (2) membranes wherein either an inside surface or an outside surface has a dense skin layer and the other is a microporous or porous layer, such as those described in, for example, Journal of Applied Polymer Science, Vol. 21, 156 (1977), JP-A-57-82515 and JP-A-58-114702 and HP series membranes made by AMICON CO.
From the standpoint of filtration resistance, the most filtration-resistant portion in these membranes is the portion which possesses minimum pore size (corresponding to the skin layer of the above-described membranes (1) and (2)). Pores formed on the surface of the membrane are of such a low pore-opening ratio (usually several % to ten-and-several %) that it is extremely disadvantageous to form portions with a minimum size on the surface in view of the filtration resistance involved. Therefore, efforts have so far been made to decrease the filtration resistance, for example, by employing a membrane structure such as one in which the thickness of the minimum pore size-layer to be formed on the surface is made extremely thin, with coarse voids being formed in the interior of the membrane. The membrane structure of membrane (2) corresponds to this type. However, this extremely thin dense layer formed on the surface is so susceptible to flaw formation during, for example, the hollow fiber-spinning step or a subsequent module-assembly step that the coarse pore size can be exposed by a slight disturbance. This destroys the reliability of the filtration. Membranes of type (1) or the like having a dense layer on both of the inside and outside surfaces have been devised to counteract this problem. However, membranes of this type have the defect that they have a high filtration resistance due to the presence of the dense layer on both the inside and the outside thereof. As a result, high water permeability is not achieved.
In a dry-and-wet filming process involving casting a film forming solution and immersing the cast film into a coagulating solution, control of the membrane structure to form a porous membrane structure is conducted by, for example, changing the formulation of the film forming solution (e.g., as disclosed in JP-A-60-41503) or changing the formulation of the coagulating bath (e.g., as disclosed in JP-A-56-126407).
However, this technique employed for controlling the structure of sheet-like membrane cannot be employed in the production of hollow fiber membranes. The reasons for this are as follows. Since supports are not used in casting hollow fiber films which is different from sheet-like films, a hollow state cannot be maintained unless the film forming solution discharged through a nozzle is rapidly coagulated by using a poor solvent with a coagulating ability as a core solution. Moreover, since all of the procedures from discharge of the film forming solution to coagulation are dynamically conducted, continuous spinning becomes impossible unless the flow rates of the film forming solution and the core solution, winding speed, etc. are well balanced, leading to frequent thread breakage and deformation of the cross section of the hollow fiber membrane.
Therefore, when employed in the spinning of hollow fiber membranes, the above-described technique which enables a porous membrane of desired structure in sheet-like membranes to be obtained does not assure formation of a porous membrane of desired structure.
With hollow fiber membranes, it is difficult to obtain a large pore size in the inside surface of hollow fiber membranes since a core solution inert to the film forming solution and exerting the same function as a support used in forming a sheet-like membrane cannot be used.
Porous membranes produced by the dry-and-wet process comprise an outside dense layer usually called a skin layer, an intermediate sparse layer called a support layer (porous layer) and an inside dense skin layer. The skin layer is a layer contributing to the separation function across the section of the membrane. The degree of denseness can be said to express the pore size of the membrane. However, a skin layer having this separation function acts in opposition to filtration efficiency, particularly filtration flow rate. That is, the skin layer exhibits such a high filtration resistance that it decreases the filtration flow rate per unit time. Hence, attempts to enlarge the pore size of both or one of the inside and outside skin layers for decreasing filtration resistance have been made. For example, JP-63-92712 discloses a process of enlarging pores in both surfaces by using a triple-ring nozzle for spinning hollow fibers. However, this process enables a pore size of only about 0.3 .mu.m to be obtained. Formation of such skin layer is considered to proceed as follows. First, when contacted with a poor solvent, a diffusion of the solvent of the film forming solution into the poor solvent begins at the interface. At the same time, diffusion of the poor solvent thereinto also begins. In general, the solvent for the film forming solution and the poor solvent used are miscible with each other, and hence their diffusion proceeds so rapidly that rapid coagulation of polymer takes place at the interface between the film forming solution and the poor solvent. As a result, a dense skin layer is first formed, and subsequent diffusion proceeds through this skin layer. Thus, the interior of the membrane has a comparatively porous structure.